Following an increase of computer and internet use in late years, a demand for information transmission is rapidly increased and a transmission system having high transmission capacity is required. In June of 2002, 10 Gigabit Ethernet (registered trademark) is standardized as IEEE (registered trademark) 802.3ae. Accordingly, commercialization of information transmission devices and information transmission medium constituting the transmission system having high transmission capacity is anticipated, and information transmission device venders and information transmission medium makers consider the improvement of the transmission capacity of the information transmission devices and the information transmission medium.
Owing to this situation, use of conventional metal cables is shifting to use of optical fiber cables as the information transmission medium. The optical fiber cable treating optical signal can perform super-long distance data communication because of lower signal attenuation than the metal cable treating electric signal. In addition, since shielding from leak of the optical signal is easier than leak of the electric signal, a large number of the optical fiber cables can be bundled without mutual interference.
As types of the optical fiber cables, there are silica glass optical fiber cables and plastic optical fiber cables and so on. Especially, the plastic optical fiber cable can be easily manufactured and processed with low cost compared to the silica glass optical fiber cable.
The plastic optical fiber cable has a plastic optical fiber (POF) as a light guide path. The POF includes a core for transmitting light and an outer shell provided around the core for keeping the light inside the core. The light entering to the core passes through the POF while repeating total reflection at the interface between the core and the outer shell having different refractive indices. Recently, a graded-index POF (GI-POF), in which the refractive index gradually increases toward the center of the core in radial direction, has gotten a lot of attention. In the GI-POF, light through the center and light entering the periphery are transmitted in approximately same time by its specific refractive index distribution (profile). Therefore, distortion does not occur in an input signal, which realizes large capacity of transmission and high-speed communication.
As a production method for the POF, there is a method in which a preform of the POF is formed and then heat-drawn to be the POF having a desired diameter. In Japanese Patent Laid-open Publication 10-96825 (by the inventor of the present invention), in a preform producing process, a polymerizable composition is poured into a pipe and the polymerizable composition is polymerized in the rotating pipe is rotated, to form a polymer layer inside the pipe. Above processes are repeated to form a preform having a plurality of polymer layers concentrically stacked. Since the polymerizable composition for each layer having a corresponding refractive index different from for other layers is used, a GI-POF having a desired refractive index distribution can be manufactured.
Meanwhile, as another solution for increasing the information transmission capacity of the transmission device, wavelength-division multiplexing (WDM), dense wavelength division multiplexing (DWDM) or the like are proposed. These methods are utilizing an optical property that light beams of different wavelengths do not interfere in each other. Since the optical signals having different wavelengths are transmitted at the same time in the optical fiber, the information transmission capacity in the optical fiber can be drastically increased.
However, the transmission system utilizing WDM or DWDM is now used only in few specific areas, because transmission devices for this system require high cost. Accordingly, to provide a transmission system having a superior transmission property with low cost, a POF having superior transmission property is needed. However, a manufacturing method for a POF having a high-bandwidth property of 10 Gbps, and a refractive index distribution for the POF serves to give the high-bandwidth property of 10 Gbps are still not disclosed.
An object of the present invention is to provide a POF having a superior transmission property and a manufacturing method thereof. Further, another object of the present invention is to provide an optical transmission system using such POF.